The storage and transmission of data representing visual images is presently a subject of much concern. There is a worldwide effort to accomplish communications networks through which data can be quickly and economically communicated to those people that require it. Some of this data is in the form of voice communications, computer readable data, and in other relatively simple configurations. However, to complete the scheme it is thought to be necessary also to include the ability to store and transmit both still and moving visual images. Systems such as a Broadband Integrated Systems Data Network ("BISDN") are, even now, being implemented to accomplish this purpose.
The purpose of these efforts is to remove the restrictions which have been imposed by limited access to data, thus making the knowledge of the world available to the people of the world at all levels, whether it be for commerce, scientific research, or merely for entertainment. Since much of this knowledge is either entirely in visual form or else requires visual accompaniment to be fully understood, the objective could obviously not be fully achieved unless transmission of visual images is included as an aspect of the system. However, it is a significant problem that visual images can include a great deal of information which, when reduced to digitized form, requires a correspondingly large amount of data to be stored and transmitted. In the case of still images, it is a problem that they take up so much digital storage space to archive and a further problem that they require so much time and/or bandwidth to transmit. In the case of moving images, the problem is much greater. Indeed, even with the great bandwidth available with proposed BISDN standards, conventional digitized video images cannot be transmitted rapidly enough to communicate moving images of reasonably high quality. Either the resolution of the image and/or the refresh rate (the rate at which the frames of the image are replaced) must be compromised using conventional prior art methods.
In an effort to solve the problem, a number of means of data compression of video images have tried. One method that is widely used is to delete portions of the data, as by eliminating every other line and/or every other pixel in a line of a scanned image. Relatively simple algorithms are used, as required, to reconstruct a whole image from such a data depleted image. Indeed, for many purposes this is an entirely satisfactory method, and the quality of the picture produced thereby is only marginally inferior to the original. More complex methods also are being tried. One such relatively complex method is to analyze each succeeding frame in a moving video image to determine just which portions of the image are changed as compared to the last. Then, information containing only the changed portion of the image is transmitted. This method can result in an image of very good quality. However, it requires a great deal of computing power, particularly at the sending end of the video data transmission, and only expensive high speed computers can be used in accomplishing this method. As can be appreciated, if it were to take longer to perform the necessary calculations than the normal transmission time of a video frame then the whole purpose of this method would be defeated, and only the relatively expensive computers can perform the necessary calculations with sufficient rapidity.
Yet another disadvantage of all of the above mentioned prior art methods of video data compression is that there is only a comparatively small reduction in the amount of data which must be stored or transmitted. To the inventor's knowledge, no prior art method has achieved an optimal amount of data compression of video images.
To the inventors' knowledge, prior to the present invention an optical solution to the above discussed problem has not been tried. However, the present inventor has realized that there exists in the prior art of photography the grain of an idea that can be applied to the present problem. Since the early days of photography, there has been much effort expended in developing and improving various means for recording images on film that can be developed into a photograph having an accurate reproduction of the colors of the original subject. At the inception of the science of color photography the efforts were divided between the search for a polychromatic film that could be altered in different ways by different light spectra, and the search for a means of encoding a photographic image so that it could be stored on a monochromatic film. With improvements in the chemical processes involved, polychromatic film proved to be the best and most practical solution to the problem for most applications, and today those skilled in the art will recognize that color photography is accomplished almost exclusively using such polychromatic film.
However, there are certain inherent advantages to the alternative approach. Among these are the fact that monochromatic film can be produced with less expense. More importantly, for many purposes, monochromatic film can be stored for extended periods of time without deteriorating, while most common "color" films fade markedly over time. Also, all other things being equal, a shorter exposure time may be used with monochromatic film, thus making it a better choice for photography in low light conditions, or to photograph fast moving subjects. For these reasons, the study of encoding color images using monochromatic media has not entirely died in the field of photography. For example, U.S. Pat. Nos. 3,586,434 and 3,609,010, both issued to Mueller, have each presented inventions relating to the encoding of color images onto a monochromatic medium. Briefly, the general techniques around which the Mueller inventions revolve (and which are common both to the Mueller inventions and to a great many other efforts in the field) involve superimposing three images on a monochromatic film, each of the three being the result of a reflection of light from an object which is spatially and spectrally filtered such that the stored image is modulated according to Fourier transform techniques accomplished by the physical means of the spatial and spectral filters. The image can be reconstructed to its original form generally by a physical process essentially the reverse of the encoding technique. This technology is sometimes referred to as "spectral zonal photography", and spectral zonal photography is a subset of the more general technology of "optically modulated imagery". The more general optically modulated imagery differs from the previously described spectral zonal photography except that the different information modulated onto the film is not restricted to being the three primary colors of a single image. Rather, using optically modulated imagery, a number entirely different objects can be encoded into an image and later separated out using Fourier transform techniques. In a work entitled Physical Optics Notebook: Tutorials in Fourier Optics written Reynolds et al. and copublished The International Society for Optical Engineering ("SPIE") and the American Institute of Physics, chapter 34 is entitled "Optically Modulated Imagery" and the known aspects of this science are discussed at length therein.
Although it is suggested in the prior art that it is feasible to use positive color filtering (using red, green and blue filters instead of the generally applied yellow, magenta and cyan filters) to separate the red, green and blue aspects of the visible spectrum and thus to produce a positive image, the utility of the prior art methods has been associated with the advantages of using black and white negative film, and thus the prior art has been concentrated on this aspect of the method. Therefore, to reconstruct an image of the original scene it has been necessary to cause the reconstructed image (decoded from the black and white film) to be projected back onto negative color film and then to develop that film using conventional techniques. Similarly, while it has also been suggested in the prior art teachings that the encoding process could conceivably be accomplished in a single operation, actual practical prior art encoding methods have involved a multi step operation. In any event, to the inventor's knowledge, none of the prior art has suggested using spectral zonal photography, or the like, to encode an image into digital format, nor have any of the prior art methods been reasonable adaptable to this purpose.
To the inventor's knowledge, no prior art means for using the advantages attainable through optically modulated imagery in the context of digitally encoded data has been devised. All prior art methods for digitally compressing video image data have been less effective or otherwise less desirable than an optical modulation method. Furthermore, all prior art optical modulation methods have required optical reconstruction techniques, which makes the prior art method unsuitable for use with commonly available computer equipment.